1. Field of the Invention
The present invention relates to a light-emitting element, a light-emitting device, a display device, an electronic appliance, and a lighting device in each of which an organic compound is used as a light-emitting substance.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By applying voltage to this element, light emission from the light-emitting substance can be obtained. In particular, a light-emitting element in which the light-emitting substance is an organic compound is referred to as an organic light-emitting diode (OLED) or organic EL and has the following features.
The above light-emitting element is a self-luminous element and thus has higher pixel visibility than a liquid crystal display and does not need a backlight. Therefore, the light-emitting element is suitable for a flat panel display element. Further, using the light-emitting element, a thinner and lighter display can be manufactured.
In addition, the light-emitting element has an extremely high response speed; accordingly, a high-quality moving image can be provided.
Further, the light-emitting element can be formed in a film form, enabling planar light emission. This is a feature which is difficult to obtain from point light sources typified by an incandescent lamp and an LED lamp or linear light sources typified by a fluorescent lamp. Accordingly, the light-emitting element has a great deal of potential for use as planar light sources which can be used for lighting and the like.
One of important properties of such a light-emitting element is the degree of luminance degradation over driving time, i.e., the lifetime. With the accumulation of the driving time, the luminance of the light-emitting element is reduced even when the light-emitting element is made to emit light at the same current density. Japan Luminaires Association has determined that the lifetime of a lighting device having such a property is the time when the luminance becomes 70% of its initial luminance; it is said that the lifetime of a commercially available LED lamp is 40000 hours to 50000 hours. An OLED lighting, which is a latecomer, needs to secure at least the same lifetime or longer. Needless to say, also in an OLED display, it is preferable that the degradation in luminance be as little as possible.
Patent Document 1 discloses a light-emitting element in which a recombination region is formed at the interface between a first light-emitting layer and a second light-emitting layer.